Method and device for producing aerogels

ABSTRACT

A method of producing an aerogel layer on a substrate is described. A precursor mixture is provided by mixing at least one material selected from the group consisting of silicates, metal alcolates, aluminates and borates with a solvent. The precursor mixture is used to form a lyosol. The precursor mixture or the lyosol formed therefrom is then applied to the substrate wherein a layer of the lyosol is formed on the substrate. A gel is formed from the lyosol by chemical conversion at a temperature at which the solvent is present in a liquid state. At a pressure between 0.5-2.0 torr the ambient temperature is reduced by about 3-70k below the point at which the solvent is converted into a solid state. The solvent is then converted into a gaseous state in a drying chamber while reducing the pressure of the solvent below the triple point and removing solvent from the gel layer.

The invention relates to a method and for producing an aerogel on asubstrate. Aerogels are highly porous materials comprising silicon ormetal oxides, which are characterised by particularly low densities of70-300 Kg/m³ at extremely high internal surface areas of up to 1000 m²/g(see DE 3924244 A1). They are produced from a lyogel, which is producedin a sol-gel process from a lyosol. Gels are shape-stable dispersesystems comprising at least two components, generally a solid,colloidally distributed material with long or heavily branched particlesand a dispersant. If the dispersant between the particles is a liquid, alyogel is present. If the liquid of the lyogel is replaced by air as thedispersant, an aerogel is produced.

A known method for producing a SiO₂ aerogel is described S. S. Kistlerin J. Phys. Chem. 36 (1932), pages 52-64. Water glass is used therein asthe precursor and acid (HCl; H₂SO₄). A hydrogel is formed in which thewater is subsequently substituted by ethanol or methanol. Thealcohol-containing gel which is produced is then subjected in anautoclave to a pressure of more than 71 bar and a temperature of >100°C. The solvent is in a state above the critical point at which there isno more surface tension. The solvent can escape from the gel withoutshrinking of the gel occurring by reason of the surface tension. Theescaping solvent is drawn off so that drying of the gel occurs. Anaerogel is produced. Of disadvantage with this so-called supercriticaldrying in an autoclave are the high pressures and temperatures whichresult in an expensive method. Methods are also known in which thealcohol is replaced by carbon dioxide, which permits supercriticaldrying at low temperatures. Furthermore, two further methods arementioned in International Patent Publication WO92/926213 A1. A firstmethod mentioned therein is freeze drying. An alcogel (ie. a lyogel withalcohol as the dispersant) is frozen so that a solid gel is produced.The solvent is subsequently removed under reduced pressure bysublimation. The publication refers to no solvent suitable for thispurpose and, furthermore, condemns this method due to a disadvantageousvolumetric increase during crystallisation or during freezing.Furthermore, the publication is concerned with a method for directlyvaporising the solvent from the liquid phase into the gas phase. Thenegative influences of the surface tension are to be minimised by asuitable solvent mixture and conduct of the method.

A method is also disclosed in publication WO95/17347 A1 which, for thepurpose of avoiding the disadvantages of the supercritical drying,proposes a sub-critical vaporisation at increased or reduced pressurewith respect to normal pressure or at ambient pressure. Thedisadvantageous effect of the surface tension of the vaporising liquidis reduced in this case also by a suitable conduct of the method and/orsuitable solvents, which exhibit a low surface tension. Although thepublication names a large number of possible solvents on an inclusivebasis, only embodiments with the solvent isoproponol, optionally withthe addition of methanol, are described in more detail.

A method of producing aerogels, which is also supposed to findapplication in the field of microelectronics, is disclosed in EP 0775669A2. A precursor comprising tetraerthoxysilane (TEOS) water andmultisolvent (a solvent mixture comprising a polyol and ethanol) isused. After deposition of the gel by spinning it onto a semi-conductorwafer, the ethanol is vaporised. After addition of a catalyst from thegas phase, gelling occurs, whereby a wet SiO₂ network is produced, thegel is then dried by vaporisation of the polyol. As a result of areaction between the polyol and TEOS, the storage and transport time ofthe precursor is heavily limited. The addition of the catalyst from thegas phase results in a vertical diffusion gradient in the aerogel layer,whereby inhomogeneous layer properties can occur in the verticaldirection.

It is an object of the invention to produce an aerogel layer withhomogeneous properties in a simple and economical manner.

This and other objects of the present invention are therein solved by amethod of producing an aerogel layer on a substrate, said methodcomprising the steps of: a) providing a precursor mixture by mixing atleast one material from a group including silicates, metal alcoholates,aluminates and borates with a solvent, said precursor mixture forforming a lyosol; b) applying the precursor mixture or a lyosol formedtherefrom to said substrate, wherein a layer of the lyasol is formed onsaid substrate; c) forming a gel from the lyosol, a temperature beingselected at which the solvent is present in a liquid state; d) reducing,at a pressure of between 0.5 and 2 bar, the temperature by about 3-70Kbelow the point at which the solvent is converted into the solid state;and e) converting the solvent into the gaseous state in a drying chamberwhilst reducing the pressure of the solvent below the triple point andremoving the solvent from the gel layer.

In the method in accordance with the invention for producing an aerogellayer on a substrate, a precursor is firstly prepared by mixing at leastone material from a group including silicates, metal alcoholates,aluminates and borates with a solvent to form a lyosol mixture.Silicates include the salts and esters of orthosilicic acid and theircondensation products, that is to say for instance, the silicic acidsand the silicon alcoholates (also referred to as silicon alcoxides oralcoxysilanes). A lyosol is a colloidal solution in which a solid orliquid substance is dispersed in a liquid medium. The addition of watercan possible be necessary to form the lyosol. The lyosol is not produceddirectly on mixing the material with the solvent but only after aninitial chemical conversion (e.g. hydrolysis and polycondensation).

The precursor mixture or the lyosol formed therefrom is then applied tothe substrate. As a result of further chemical conversion (hydrolysisand polycondensation), a gel is formed from the lyosol, whereby atemperature is selected at which the solvent is present in a liquidstate. The pressure is preferably so selected that it is between 0.5 and2 bar where the range is include, ambient pressure and a slightlyreduced pressure. The pressure range may be adjusted relatively easilyfrom the process engineering point of view. The temperature is soselected that the solvent is present in a liquid state in order topromote the formation of a gel.

After the has been formed (whereby small accounts of the startingmaterials and products of the reactions can remain in the gel), thetemperature is reduced, at a pressure between 0.5 and 2 bar, by about2-70K, preferably 5-15K, below the point at which the solvent changesinto the solid state (solidifies). The only slight reduction oftemperature below the solidification point results, on the one hand, ina saving of energy and an acceleration of the method and, on the otherhand, in low mechanical stresses in the gel layer induced by thetemperature and solidification.

The solvent is then converted into the gaseous state in the dryingchamber whilst reducing the pressure of the solvent to below the triplepoint and removed from the gel layer. The gaseous solvent is conductedout of the drying chamber. When reducing the pressure, the temperaturecan be maintained constant or increased. Of importance is merely thatthe change of state from the solid into the gaseous state occurs withoutpassing through the liquid state, ie. below the triple point.

Preferably a solvent is used which has a triple point at temperatureabove 0° C., preferably above 15° C. This permits a considerablesimplification of the apparatus since cooling water can be used forcooling purposes.

In an advantageous embodiment of the method in accordance with theinvention, a low-molecular tertiary alcohol is used as the solvent.Low-molecular tertiary alcohols, particular t-butanol, have, on the onehand, a triple point at a relatively high temperature. On the otherhand, tertiary alcohols are less reactive and have a smaller tendency toalcoholysis. They thus have a better compatibility with a largeproportion of the group of materials including the silicates, whichresults in better transportability and storage ability of the precursormixture. Furthermore, their evaporation properties permit rapidsublimation.

In a preferred embodiment of the method in accordance with the inventiona precursor is prepared by mixing a material predominantly comprising analcoxysilane with a low-molecular tertiary alcohol, constituting thesolvent, and with water. An aerogel substantially comprising siliconoxide is formed from such a precursor. In addition to the predominantproportion of the alcoxysilane, the mixture can include, for instance, asmall proportion of a metal alcoholate or borate in order to modify thesilicon oxide structure. The addition of water serves to hydrolyse thealcoxysilane, whereby the products which are produced cross-link bypolycondensation. Although water is formed again during the latter, alarger amount is consumed during the hydrolysis.

Tetraethoxysilane (TEOS) is preferably used. T-butanol is preferablyused as the low-molecular tertiary alcohol. Tetraethoxysilane is aneconomic starting material, which is frequently used withinsemi-conductor technology and which furthermore, is not so toxic as eg.tetramethoxysilane (TMOS). It has transpired that the combination oftetraethoxysilane with t-butanol results in a precursor with anexcellent storage ability and outstanding processing qualities,associated with which, amongst other things, is a relatively smallhealth risk. The combination of materials also has a good miscibilitywith water, which is required as a reaction partner for the hydrolysisof the tetraethoxysilane.

The water, which is mixed into the precursor in addition to the solventin the preferred embodiment, acts principally as a reaction partner forthe hydrolysis as a prerequisite for the sol-gel method. The precursoris preferably prepared by firstly mixing a material from the group ofmaterials referred to above with the solvent, whereby the solvent(possibly solid at room temperature) is liquefied by heating before themixing. The water is then mixed in. A catalyst is preferably added toaccelerate the gel formation. Both multi-stage catalysis methods, inwhich, initially, a first catalyst (eg. NH₄OH) which promotes thehydrolysis and, subsequently a second catalysis (eg. HCl), whichpromotes the condensation are added and the addition of only onecatalyst (eg. HF), which accelerates both reactions, are possible. Theuse of a catalyst, which is preferably added in an aqueous solution,accelerates The method and thus increases its efficiency.

A preferred embodiment of the method in accordance with the invention ischaracterised in that a pre-gelling occurs before the application of thelyosol to the substrate. This increases the viscosity of the substanceso that relatively thick layers of the gel can be produced with simple,conventional application methods, for instance, by spinning.

In a preferred embodiment of the method in accordance with theinvention, the gel layer formed on the substrate is washed with thesolvent before the reduction in the temperature to solidify the solvent.This serves to enrich the solvent in the gel layer and, at the sanetime, to remove excess starting materials of the hydrolysis andcondensation reactions. A hydrophobic agent can advantageously be addedat the same time to the washing process, which replaces OH groups curdwater in the gel layer by alkyl groups (eg. hexamethyldisilazane ortrimethylchlorosilane).

A preferred embodiment of the method in accordance with the invention ischaracterised in that a precursor is prepared by mixingtetraethoxysilane with more than twice the amount (moles) of t-butanol,preferably with 4 to 30 times the amount of t-butanol, at a temperatureat which the mixture is present in liquid form and water is added in anamount which corresponds to 4 to 30 times, preferably about 6 to 10times the amount of the tetraethoxysilane. A catalyst which accelerateshydrolysis and/or condensation is then added. Before application to thesubstrate, the mixture is then stored at a temperature at which themixture reimains liquid and under an atmosphere saturated witht-butanol. The pre-gellification takes place which increases theviscosity. Depending on the choice of catalyst, the mixture ispreferably stored for a period between 1 hour and a number of days.Catalysts are also possible which permit a shorter time. The pre-gelledmixture is then applied to the substrate by spinning and caused tosolidify by reducing the temperature. These steps preferably occur atpressures between 0.5 and 1 bar. The t-butanol is finally converted to agaseous state below the triple point by reducing the pressure to below0.05 bar and is, thus, removed from the gel layer. The gaseous t-butanolis conducted out of the drying chamber. After the drying process, theaerogel layer applied to the substrate is post-treated at a temperaturebetween 200° C. and 800° C. in an inert gas atmosphere or a reducedpressure air atmosphere. Both the residue of the water and also residuesof undesired excess starting materials are, thus, removed. A treatmentwith a hydrophobia-imparting agent (e.g. hexamethyldisilazane ortrimethylcliorosilane) is then preferably performed in order to replaceremaining OH groups (or water) with methyl or other alkyl groups. Thistreatment can be performed in the gas phase.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below with reference toexemplary embodiments and associated drawings, in which:

FIG. 1 is a block illustration of an installation for producing theaerogel layer;

FIG. 2 is a basic illustration of a deposition and gelling module;

FIG. 3 is a diagram illustrating the aerogel layer thicknessdistribution on a silicon wafer;

FIG. 4 is a phase diagram of the solvent which is used; and

FIG. 5 is a diagram which shows the improvement in the switching timewhen using an aerogel layer as the dielectric in comparison to an SiO₂layer.

FIG. 1 is a block view of an installation for producing an aerogel layeron a substrate. The installation comprises a mixing module 1, adeposition module 2, a gelling module 3, a drying module 4, and apost-treatment module 5. A preferred method for producing the aerogellayer will be described below in which The individual method steps areperformed successively in the different modules shown in FIG. 1. Inalternative embodiments, a plurality of modules can be combined, ie. aplurality of method steps performed at one location or in one module.

A precursor is firstly prepared in the mixing module. Preferably, thethree components, tetraethoxysilane, t-butanol (tertiary butanol) andwater are firstly mixed. A predetermined amount of the t-butanol, whichis present in solid form at room temperature, is measured and heated toliquefaction. The heated liquid t-butanol is subsequently mixed with apredetermined amount of TEOS. A predetermined amount of water is mixedin at the same time or subsequently. About 4 to 30 times the amount (inmoles) t-butanol and 4 to 30 times the amount of water can be mixed intoone part of TEOS. In order to accelerate the hydrolysis and condensationprocesses, a catalyst is subsequently added. Since the catalyst is addedin aqueous solution, the amount of water that is thereby added, must betaken account of when determining the previously added amount of water.Two types of catalyst are currently preferred. One method represents atwo-stage catalysis, in which an ammonium hydroxide solution is firstlyadded to obtain a pH value of >10 of the mixture, whereby the hydrolysisis accelerated. After a predetermined hydrolysis reaction time, a pHvalue of <2 is set by addition of a second catalyst in the form of anacid (HCl), whereby the polycondensation is accelerated and proceedspreferentially. In an alternative method, after the mixing of the TEOSwith the t-butanol and the water, only one catalyst is added forinstance in the form of a 40% hydrofluoric acid (HF) to acceleratehydrolysis and polycondensation. The addition of HNO₃ is also possible.

In the two stage catalysis, the following amounts are preferred: 1 Molarpart TEOS is mixed with 10 parts t-butanol and 8 parts water 10 vol. %0.1 N NH₄ OH solution is firstly added to this mixture. In the secondstage 0.5 vol. % HCl solution is added. After the addition thereof, theratio of TEOS:water:t-butanol=1:15.3:10. For 100 ml sol, for instance,15.9 g TEOS is mixed with 56.5 g t-butanol and 11 g water; 10 vol. % (10ml) of the first catalyst (0.5 N N₄ OH) and then 0.5 vol. % (0.5 ml) ofthe second catalyst (HCl, 32%) are then added. Such a two-stageformulation may also be produced if 21.37 g t-butanol is mixed with21.37 ml TEOS and 10.37 ml water is added to this mixture. 0.6 ml N NH₄OH and 1.7 ml HCl can then be added.

After the addition of the second catalyst, pre-gelling takes place (eg.over five days at ca. 30° C.). Then the sol-gel substance formed in thepre-gelling process is deposited by means to spinning in the depositionmodule 2 (FIG. 1) onto a substrate (for instance, a silicon wafer). Thesubstrates, thus coated, move into the gelling module 3, where theyremain for five further days, for instance, at temperatures around 30°C., for the gelling and ageing. The substrates with the formed gel layerthen move into the drying module which comprises a pressure chamber. Thetemperature of the gel layer is firstly reduced therein to a temperaturewhich is below the solidification point. The pressure can be maintainedat ambient pressure or reduced slightly. After solidification of the gellayer, the pressure in the drying chamber is reduced to below 0.05 bar,whereby a change in phase of the solvent (t-butanol) takes place belowthe triple point from the solid to the gaseous state. The gaseoust-butanol escapes from the gel layer and is conducted out of the dryingchamber. This drying lasts, for instance, one hour.

The substrates with the formed aerogel are than transferred into apost-treatment module 5 where they are tempered, for instance, for 30minutes to 3 hours, at a temperature of between 200° C. and 800° C.under reduced pressure or in an inert gas atmosphere (eg. nitrogen).

With this two-stage catalysis method and the stated technologicalparameters, the following layer properties, for instance, were obtainedon a silicon wafer:

Relative dielectric constant ∈_(r)<1.7

Internal surface area A_(I) 500 m²/g

Surface roughness—Peak-to-valley height R_(a)<8 nm over an area of (200μm²)

Refractive Index circa ca 1.12 to 1.15

Two alternative embodiments of the method in accordance with theinvention use a one-stage catalysis, ie. a single addition of acatalyst. Two exemplary mixtures will be given for this:

EXAMPLE 1: Sol S

100 ml sol TEOS:water:t-butanol 22.8 g:14.8 g:47.9 g + 0.2 vol. % (=0.2ml) catalyst (HF, 40%) corresponds to the molar ratios of:TEOS:water:t-butanol before the addition of the water-containingcatalyst: 1:7.45:5.87 after addition of water-containing catalyst:1:7.55:5.87

EXAMPLE 2: Sol B

100 ml sol TEOS:water:t-butanol 13.1 g:8.1 g:61.4 g + 1 vol. % = (1 ml)catalyst (HF 40%) Molar amount ratios: TEOS:water:t-butanol beforeaddition of the water-containing catalyst: 1:7.13 after addition of thewater-containing catalyst: 1:5.5:13

The process times could be further reduced in the methods with singlestage catalysis. After the mixing and addition of the catalyst,pre-gelling of the sol takes place over about 90 minutes at atemperature of, for instance, 32° C. The layer is then deposited on tothe substrate by means of spinning. The gelling and ageing lasted about90 minutes at a temperature of c. 32° C. It is then cooled down to atemperature of about 15° C. After solidification of the gel layer,drying occurs by sublimation of the t-butanol at about 15° C. and apressure of ≦0.05 bar, which takes a time of about 30 minutes to 1 hour.The gel layer is then post-treated at a temperature of 200° C. to 800°C. at reduced pressure in an inert gas atmosphere. The followingproperties of a layer applied to a silicon wafer are achieved, forinstance, with these technological parameters:

Porosity—66% air and 34% solid material

Chemical composition Si:O:H=1:2:0.28

Pore size <10 nm

Achievable layer thicknesses—200-600 nm

Refractive index <1.2

In addition to the separate construction of the modules shown in FIG. 1,it is also possible to combine the number of modules, ie. to perform anumber of method steps in one process chamber. FIG. 2 shows anarrangement in which the mixing module 1, deposition module 2 andgelling module 3 are combined. The mixing module 1 is located above aprocess chamber. In the process chamber there is a support table (waferchuck) 6, onto which a substrate 7 (wafer; silicon disc) is placed. Apredetermined amount of the sol-gel mixture is applied to the substrate7 by means of the mixing device 8. A uniform layer is then produced onthe substrate by spinning, ie. by rotation of the support table 6.Situated in the process chamber is a device 9 for evaporating the sol(t-butanol). Gates 10.1 and 10.2 for loading and removing the substrateare provided in the wall of the process chamber.

The diagram of FIG. 3 illustrates the layer thicknesses measured onthree exemplary aerogel layers on 100 nm silicon wafers, at fivepositions in each case. TEOS and t-butanol were used to produce theprecursor in the illustrated samples. It is apparent from the diagramthat the very uniform layer thickness of the aerogel layer may beachieved with the aforementioned method. This permits the use of themethod in a process for manufacturing integrated circuit chips which areto be formed in a large number on the substrate. The uniform layerthickness results in small variations between the chips distributed onthe substrate. More important, however, is the even smaller variationwithin the chips so that the elements produced on a chip have the sameparameters.

FIG. 4 is a pressure-temperature phase diagram of the solvent which isused, for instance t-butanol. The triple point for t-butanol is at apressure of p_(i)=0.053 bar and a temperature θ_(t)=24.96° C. Duringmixing and gelling, a liquid state is adopted: the temperature is belowthe triple point temperature and the pressure is for instance, 1 bar.After the gelling has been completed, the temperature is reduced at theprevailing pressure so that it is about 3-70K, preferably 5-15K, belowthe solidification point. For instance, the temperature is reduced toabout 10-20° C. After solidification of the t-butanol, the pressure isreduced, whereby a point is approached which is situated in the gaseousregion of the phase diagram of FIG. 4. The straight line extending fromthe point, which was previously approached and is situated in the solidregion, and the point in the gaseous region intersects thesolidification boundary below the triple point so that the solventsublimes. Depending on the starring point in the solid region, when thepressure is reduced, the temperature can also be slightly increased.

It is possible with the method in accordance with the invention to applyhomogenous aerogel layers to substrates. The silicon dioxide aerogellayers produced by using alcoxysilanes particularly TEOS, are suitableas dielectric layers with a very low relative dielectric constant(∈_(r)<2). The dielectric constant of the layer thus comes very close tothat of air. The reduction of the dielectric constant can be used for areduction in parasitic capacitance of metallic conductors extending onor below the aerogel layers. The reduced parasitic capacitance resultsin reduced signal time delays in high frequency applications and also areduction in the signal crosstalk of closely adjacent conductive tracks.The aerogel layers thus produced are temperature resistant up to 700° C.

FIG. 5 is a diagram which shows the percentage switching timeimprovement when using silicon oxide aerogel layers as the dielectricbetween aluminium coatings of integrated circuits in comparison toconventional silicon dioxide layers. The relative dielectric constant ofthe aerogel is estimated at 1.7 and that of the silicon dioxide 3.9. Thespecific resistance of the aluminium is estimated at 3.7 μΩ cm. Furtherparameters were as follows:

Design values 0.18 μm (CMOS-Technology)

Driver resistance 50Ω, and

Load capacity 5fF.

As a result of the aforementioned properties of the aerogels, they canalso be used, in addition to the aforementioned usages, in the followingfields of application in which materials with large internal surfaceareas are of advantage:

Gas sensors,

Storage of catalysts (e.g. for microreactors),

Thermal insulation,

Background illumination for LCD screens,

Coating of large surfaces for the production of thermally protectedglass.

What is claimed is:
 1. A method of producing an aerogel layer on asubstrate, said method comprising the steps of: a) providing a precursormixture by mixing at least one material selected from the groupconsisting of: silicates, metal alcoholates, aluminates and borates witha solvent, said precursor mixture for forming a lyosol; b) applying theprecursor mixture or the lyosol formed therefrom to said substrate,wherein a layer of the lyosol is formed on said substrate; c) forming agel from the lyosol by chemical conversion, a temperature being selectedat which the solvent is present in a liquid state; d) reducing, at apressure of between 0.5 and 2 bar, the ambient temperature by about3-70K below the point at which the solvent is converted into the solidstate; and e) converting the solvent into the gaseous state in a dryingchamber whilst reducing the pressure of the solvent below the triplepoint and removing the solvent from the gel layer.
 2. The method ofclaim 1, wherein a solvent is used which has a triple point at atemperature above 0° C.
 3. The method of claim 2, wherein a solvent isused which has a triple point at a temperature above 15° C.
 4. Themethod of claim 1, wherein the solvent comprises a tertiary alcohol. 5.The method of claim 4, wherein the solvent comprises t-butanol.
 6. Themethod of claim 1, wherein the step of providing the precursor mixturecomprises the step of mixing a material, comprising an alcoxysilane,with a tertiary alcohol as a solvent and with water.
 7. The method ofclaim 6, wherein said alcoxysilane comprises tetraethoxysilane (TEOS).8. The method of claim 7, wherein said tertiary alcohol comprisest-butanol.
 9. The method of claim 1, wherein the step of providing theprecursor mixture comprises the step of adding water in addition to thesolvent.
 10. The method of claims 1, wherein the step of providing theprecursor mixture comprises the steps of initially mixing a materialfrom the group with the solvent and subsequently adding water, whereinthe solvent is liquefied by heating before said mixing.
 11. The methodof claim 1, wherein the step of providing the precursor mixturecomprises the step of adding a catalyst for accelerating the gelformation.
 12. The method of claim 11, comprising, before the step ofapplying the precursor mixture to the substrate, the step ofpre-gelling.
 13. The method of claims 1, comprising, after step c) andbefore step d), the step of rinsing the gel layer applied to thesubstrate with the solvent.
 14. The method of claim 13, wherein in therinsing step, a hydrophobing agent is added which replaces water and OHgroups in the gel layer by alkyl groups.
 15. The method of claim 1,wherein the step of providing a precursor mixture in step a) comprisesthe steps of: a1) mixing an alcoxysilane with a tertiary alcohol at atemperature at which the mixture is present in liquid form and addingwater; a2) adding a catalyst which accelerates hydrolysis and/orcondensation; and a3) storing the mixture at a temperature at which themixture is present in liquid form and under an atmosphere which issaturated with the low-molecular tertiary alcohol so that pre-gellingtakes place.
 16. The method of claim 15, said storing step comprisingstoring the mixture for a period of between 1 hour and a number of days.17. The method of claim 15, wherein the pre-gelled mixture formed instep a3) is applied in step b) to the substrate by spinning on.
 18. Themethod of claim 15, wherein in step a1) tetraethoxysilane (TEOS) ismixed with more than twice the amount (moles) of t-butanol.
 19. Themethod of claim 18, wherein tetraethoxysilane is mixed with 4-30 timesthe amount of t-butanol.
 20. The method of claim 18, wherein water isadded in an amount which corresponds approximately to twice to 30 timesthe amount of the tetraethoxysilane used in step a1).
 21. The method ofclaim 20, wherein water is added in an amount which corresponds to 4-30times the amount of the tetraethoxysilane.
 22. The method of claim 1,wherein steps a) and b) are performed at pressures between 0.5 and 1bar.
 23. The method of claim 22, wherein t-butanol is used as thesolvent and the pressure is reduced to below 0.05 bar in step e). 24.The method of claim 1, wherein after the removal of the solvent from thegel layer in step e): f) the aerogel layer applied to the substrate istempered at a temperature between 200° C. and 800° C. under an inert gasatmosphere or a reduced pressure.
 25. The method of claim 24, whereinafter the tempering in step f): g) the gel layer is treated with ahydrophobia-imparting agent which replaces OH groups by alkyl groups.